CA-140545: wrapper should exit after launching vncterm

To avoid an instance of fe hanging around for each vncterm we launch,
fork vncterm and exit rather than calling exec.

This means that the script has to write the child pid ($!) rather than
its own pid ($$) to xenstore.
Just because that's how it used to work...but you're right, the exec is pretty pointless since xenstore-write should just return (and I don't think we really care about XIU anyway).

I'll remove the exec, and switch the order of the two xenstore-writes for added symmetry :)